1. Technical Field
The concepts described herein relate generally to reducing magnetic field inhomogeneities for magnetic resonance imaging.
2. Discussion of Related Art
Magnetic resonance imaging (MRI) is a technique used frequently in medical and research applications to produce images of the inside of subjects such as humans and animals. MRI is based on detecting nuclear magnetic resonance (NMR) signals, which are electromagnetic waves emitted by atomic nuclei in response to excitation by an electromagnetic field. In particular, magnetic resonance (MR) techniques involve detecting NMR signals produced upon the re-alignment of the nuclear spins of atoms in the subject's tissue.
During an MRI procedure, NMR signals emitted from a volume of interest or from a slice (i.e., a relatively thin region) of the volume of interest are detected. The detected NMR signals may then be utilized to produce a two-dimensional (2D) image of the slice. A 2D image of a slice is composed of pixels, each pixel having an intensity (e.g., a magnitude or value) that is proportional to the strength of the NMR signal emitted by a corresponding location in the volume of interest. A plurality of such 2D images reconstructed from NMR signal data obtained from successive slices may be stacked together to form a three-dimensional (3D) image. A 3D image is composed of voxels,  each voxel having an intensity proportional to the strength of the NMR signal emitted from a corresponding portion of the volume of interest.
To obtain NMR signals, a static magnetic field B0 is applied to a region of interest, and nuclei within the region are excited by applying RF electromagnetic radiation at the Larmor frequency. The Larmor frequency is the frequency at which nuclear spins process about the axis of the static magnetic field B0, and is proportional to the strength of the static magnetic field B0. When applied, the RF electromagnetic radiation at the Larmor frequency causes the nuclear spins to change orientation, such that the spins are no longer aligned with the static magnetic field B0. The nuclear spins then gradually re-realign with the static magnetic field B0, releasing electromagnetic energy at the Larmor frequency that is detectable as NMR signals. Accordingly, the NMR signals contain information that is significantly dependent on the static magnetic field B0. The NMR signals may be detected using one or more RF coils sensitive to electromagnetic changes caused by the NMR signals.
Inhomogeneities in the applied magnetic field B0 may arise in various subjects, such as animals and humans, and may be caused by boundaries, such as tissue-air boundaries which cause disruptions in the magnetic field B0. Since the Larmor frequency is proportional to the magnetic field B0, inhomogeneities in the magnetic field B0 may cause the Larmor frequency to be shifted in some areas. Thus, the RF electromagnetic radiation emitted from these areas may be shifted from the expected Larmor frequency, and this electromagnetic radiation may not be detected as well as electromagnetic radiation emitted at the expected Larmor frequency. The NMR signals that are detected as a result of such field inhomogeneities may lead to undesirable artifacts in images constructed from such NMR signals.
Conventional techniques for homogenizing the magnetic field B0 include using active or passive compensation components commonly referred to in the relevant arts as “shims.” One example of an active shim is an electromagnetic coil placed in the static magnetic field B0. The electromagnetic coil may have a controllable current that induces changes in the magnetic field around the coil. However, active shims may be limited to  providing relatively coarse, low-order magnetic field corrections. A passive shim is a piece of magnetic material placed in the static magnetic field B0 that alters the field around the shim. However, image artifacts may remain in spite of these conventional techniques, as they are only partially effective in reducing the magnetic field B0 inhomogeneities.